Six sero-negative, culture-negative young adult chimpanzees were experimentally-infected with Mycoplasma pneumoniae. Each chimpanzee became colonized with peak titers of 106 to 108 ccu/ml. The oropharyngeal and tracheal tissues remained colonized for up to 70 days and lungs for 26 days. Seroconversion developed within two weeks post-challenge; the titers peaked between 1:256 and 1:1024, and antibody titers persisted throughout the study period. Positive cold agglutinin titers were detected at day 12 to 15 post-inoculation, and the response persisted for up to 40 days with peak titers ranging from 1:160 to 1:640. Animals developed overt signs of disease that corresponded with peak lung colonization, the development of positive x-ray findings and cold agglutinin titers. The infected chimpanzees developed a persistent cough and low grade fever, and some developed rhinitis, oropharyngitis, diarrhea, and lot of appetite. The specific IgA immunoglobulin values in the lung lavage fluids were 2 to 3 times greater than the IgG values. The two non-inoculated chimpanzees that served as negative controls became infected during the course of study, but we were unable to determine the mode of transmission. The experimentally-induced pneumonia in chimpanzees were remarkably similar to naturally occurring primary atypical pneumonia in humans, and this animal best reflects and parallels the pneumonia in humans, and this animal best reflects and parallels the pneumonia in humans.